Hideki Nakano*, Hiroaki Matsunaga, Hiroaki Okamoto, Makoto Okazaki

ABSTRACT: Acoustic telemetry was used to identify the short-term horizontal and vertical movement patterns of the bigeye thresher shark Alopias superciliosus in the eastern tropical Pacific Ocean during the summer of 1996. Two immature female
sharks, 175 and 124 cm PCL (precaudal length), were tracked for 96 and 70 h, respectively, demonstrating very distinct crepuscular vertical migrations similar to those reported for the megamouth shark. The bigeye threshers stayed at 200 to 500 m depth
during the day and at 80 to 130 m at night. The deepest dive extends the known depth distribution of the species to 723 m. No Œfly-glide¹ behavior (rapid ascents followed by slower acute-angled descents) was observed for the 2 sharks. However, the
opposite behavioral pattern of slow ascents and relatively rapid descents during the night was observed. Since bigeye threshers have large eyes extending upwards onto the dorsal surface of the cranium, it may be more efficient for them to hunt prey which
are highlighted against the sea surface from below. Estimated mean swimming speed over the ground ranged from 1.32 to 2.02 km h-1, similar to swordfish and megamouth sharks, and slower than that reported for tunas, billfishes, and other pelagic
sharks.